1. Field of the Invention
The present invention relates to an oil pump rotor employed in an oil pump which takes in and expels a fluid according to changes in the volume of a plurality of cells which are formed between the pump's inner and outer rotors.
Conventional oil pumps are provided with an inner rotor to which n (where n is a natural number) outer teeth are formed, an outer rotor to which n+1 inner teeth are formed for engaging with the outer teeth of the inner rotor, and a casing in which an intake port for taking in fluid and an discharge port for discharging fluid are formed. In this oil pump, the inner rotor is rotated, causing the outer teeth to engage with the inner teeth, and thereby rotate the outer rotor. Fluid is taken in or expelled from a plurality of plurality of cells formed between the two rotors due to changes in the volume of the cells.
Individual cells are partitioned due to contact between the respective outer teeth of the inner rotor and the inner teeth of the outer rotor at the front and rear of the direction of rotation, and by the presence of the casing of the oil pump at either side of the inner and outer rotors. As a result, independent fluid carrier chambers are formed. Once the volume of a cell has fallen to a minimum value during the process of engagement between the outer teeth of the inner rotor and the inner teeth of the outer rotor, the cell next proceeds along an intake port where its volume is expanded, causing fluid to be taken up. After the cell's volume reaches a maximum value, the cell next proceeds along an discharge port where its volume is decreased, causing the fluid to be expelled.
Because of its small size and simple structure, an oil pump of this design has wide applications, including use as a lubricating oil pump in automobiles, an oil pump in automatic transmissions, and the like. When such oil pumps are installed in automobiles, a drive means therefore is provided by directly attaching the inner rotor to the engine's crank shaft, so that the oil pump is driven by the rotation of the engine.
In order to reduce noise generated by the pump while at the same time improve mechanical efficiency, oil pumps of the above design are provided with a suitably large tip clearance between the tips of the teeth of the inner and outer rotors at a position which is rotates by 180.degree. from the position of engagement of the teeth in the assembly of the inner and outer rotors.
Various means may be proposed for securing the tip clearance, including providing clearance between the respective surfaces of the teeth of the rotors by carrying out uniform run-off, so that tip clearance is secured between the tips of the teeth on each of the rotors during engagement. Alternatively, tip clearance may also be secured by flattening the cycloid curve.
The oil pump disclosed in Japanese Patent Application, First Publication No. Hei 5-256268 is a so-called cycloid pump, in which the tips of the teeth of the pinion (inner rotor) and the tooth spaces of the internally toothed ring gear (outer rotor) have an epicycloid shape generated by rotating a first cycloid generating circle on the pitch circle of the pinion and the internally toothed ring gear; and the tooth spaces of the pinion and the tips of the teeth of the internally toothed ring gear have a hypocycloid shape generated by rotating a second cycloid generating ring on the pitch circle of the pinion and the internally toothed ring gear (the radius of the first cycloid generating circle is different from the radius of the second cycloid generating circle). In this oil pump, two rotating circles are used to form the tooth profile of the pinion and the internally toothed ring gear, so that the tips of the teeth of the pinion and the tooth spaces of the internally toothed ring gear are generated by the same first cycloid generating circle, and the tooth spaces of the pinion and the tips of the teeth of the internally toothed ring gear are generated by the second cycloid generating circle.
In the pump disclosed in the above reference, in order to reduce the noise generated by the pump and improve its mechanical efficiency, two cycloid curves are flattened to an extent that corresponds to the required radial clearance between the tips of the teeth in the area opposite the point where the pinion and the internally toothed ring gear engage most deeply, and so that the clearance at the point where the pinion and the internally toothed ring gear most deeply engage is significantly reduced. As a result, the pulsation of the relayed fluid is greatly reduced, and improvements are realized with respect to the noise generated by the pump, and the pump's mechanical efficiency and durability.
Incidentally, in the pump disclosed in the aforementioned reference, a closed cycloid curve is generated by connecting with a straight line the beginning and end points of a flattened cycloid curve, and the beginning and end points of an non-flattened cycloid curve on the pitch circle. However, there is the possibility that engagement between the pinion and the internally toothed ring gear will not be carried out smoothly, due to the generation of a straight line component in one portion of the cycloid curve. For example, during the process of movement of the tips of the teeth of the pinion move along the surface of the tooth spaces of the internally toothed ring gear from the position of engagement between the pinion and the internally toothed ring gear, a deflection may occur when the tips of the teeth of the pinion move from the curved line portion to the straight line portion, or from the straight line portion to the curved line portion, thus interfering with smooth progression of the engagement.
2. Description of the Related Art
The present invention was conceived in consideration of the above-described problems, and has as its objective an improvement in the mechanical efficiency and efficiency of an oil pump, by providing a suitably large interval of space between the tips of the teeth of the inner rotor and the tooth spaces of the outer rotor during the engagement of the rotors, thereby reducing the sliding resistance between the surfaces of the rotor teeth.
In order to meet the above-state objectives, in the oil pump rotor of the present invention, the inner rotor is designed such that the profile of the tips of the teeth thereof is prescribed by an epicycloid curve generated by a first outer rotating circle which circumscribes the base circle of the inner rotor and rotates without slipping along the base circle of the inner rotor, and the profile of the tooth spaces is prescribed by a hypocycloid generated by a first inner rotating circle which inscribes the base circle of the inner rotor and rotates without slipping along the base circle; and the outer rotor is designed such that the profile of the tooth spaces is prescribed by an epicycloid generated by a second outer rotating circle which circumscribes the base circle of the outer rotor and rotates without slipping along the base circle of the outer rotor, and the profile of the tips of the teeth is prescribed by a hypocycloid curve generated by a second inner rotating circle which inscribes the base circle of the outer rotor and rotates without slipping along the base circle of the outer rotor. When the diameters of the base circle, first outer rotating circle, and first inner rotating circle of the inner rotor are designated as bi, Di, and di, respectively, and the diameters of the base circle, second outer rotating circle, and second inner rotating circle of the outer rotor are designated as bo, Do, and do, and the eccentric load of the inner and outer rotors is designated as e, then the inner and outer rotors are formed to satisfy the following: EQU bi=n.multidot.(Di+di), bo=(n+1).multidot.(Do+do) EQU Di+di=Do+do=2e EQU (n+1).multidot.bi=n.multidot.bo EQU and, EQU Do&gt;Di, di&gt;do
It is preferable to form the inner and outer rotors to satisfy the expression: EQU Di+t/2=Do, di-t/2=do
where t (where t.noteq.0) indicates the size of the space between the tips of the teeth on the outer rotor and the tips of the teeth on the inner rotor.
It is preferable to form the inner and outer rotors of the oil pump rotor of the present invention such that: EQU 0.03 mm.ltoreq.t.ltoreq.0.25 mm (mm: millimeter)
It is preferable to form the oil pump rotor of the present invention to satisfy: EQU 0.850.ltoreq.Di/Do.ltoreq.0.995
As a condition necessary for determining the tooth profile of the inner and outer rotors, the rotating distance of the first outer rotating circle and the first inner rotating circle of the inner rotor must be closed in one circumference, i.e., must be equal to the circumference of the base circle of the inner rotor. Thus, EQU bi=n.multidot.(Di+di)
Similarly, the rotating distance of the second outer rotating circle and the second inner rotating circle of the outer rotor must be equal to the circumference of the base circle of the outer rotor. Thus, EQU bo=(n+1).multidot.(Do+do)
Next, since the inner and outer rotors engage, EQU Di+di=Do+do=2e
From the above equation, EQU (n+1).multidot.bi=n.multidot.bo
such that the tooth profiles of the inner and outer rotors are formed to satisfy the preceding equation.
In the oil pump rotor formed to satisfy the preceding condition, when EQU Do&gt;Di, di&gt;do
then, it is possible for the profile of the tips of the teeth of the inner rotor, formed by the first outer rotating circle Di with respect to the profile of the tooth spaces of the outer rotor formed by the second outer rotating circle Do, and the profile of the tips of the teeth of the outer rotor, formed by the second inner rotating circle do with respect to the profile of the tooth spaces of the inner rotor formed by the first inner rotating circle di, to secure a larger backlash between the surfaces of the teeth of both rotors during engagement as compared to the conventional technologies. "Backlash" is the gap during engagement which is attainable between the tooth surface of the inner rotor which is positioned opposite the tooth surface which applies the load and the tooth surface of the outer rotor which opposes the aforementioned surface of the inner rotor.
The above relational equations must also be established in the case where the tooth profiles of each of the rotors are formed to provide tip clearance. Therefore, the necessary tip clearance t is equally divided between the rotor engagement position and the opposing position of the tips of the teeth of each of the rotors (i.e., the position where tip clearance has been provided). This will be referred to as "clearance" hereinafter. Tip clearance t is split between the tooth surfaces of the rotors at each position. This clearance can be secured by employing the following relational equations. EQU Di+t/2=Do, di-t/2=do
Two clearances (t/2) are produced at the rotor engagement position and the position of opposing tooth-tips, respectively. When the rotors are assembled, the clearance at the engagement position shifts to the position of opposing tooth-tips, so that tip clearance t is formed between opposing tooth-tips.
The inner and outer rotors of the oil pump rotor of the present invention are formed so that the profile of the tips of the teeth on the inner rotor is slightly smaller than the profile of the tooth spaces of the outer rotor, and the tooth profile of the tooth spaces of the inner rotor is slightly larger than the profile of the tips of the teeth of outer rotor. Therefore, it is possible to set the backlash and the tip clearance to be suitably large. As a result, as compared to the conventional technology, a relatively larger backlash can be secured while keeping the tip clearance small. Thus, it is difficult for a pressure pulsation to occur in the fluid, while the sliding resistance between the tooth surfaces of the rotors is reduced.